This invention relates to a rotary device such as a rotary gear pump in which the inner rotor has an outer tooth profile in the shape of the inner envelope of an epitrochoidal curve.
In a known rotary pump, the outer rotor has a tooth profile whose cross section is defined by a plurality of circular arcs of same radius of curvature centered at equal intervals on a circle of larger diameter defining the respective outer rotor teeth, the circular arcs being located inside the larger circle, and the adjacent circular arcs being connected by radially outwardly curved arcs inside the larger circle.
The inner rotor of this known rotary device has an outer profile in the shape of an inner envelope of an epitrochoidal curve. In an effort to obtain maximum discharge from within the limited volume available between the inner and outer rotors while minimizing pulsation of the discharge and/or reducing cavitation under high speed rotation, prior pumps of this type have been designed with a balancing of maximization of the number of teeth and minimizing the space taken up by the teeth so as to leave as much space as possible for the discharge. This has resulted in a tooth profile defining narrow and/or more sharply angled teeth.
The inventor has found that the parameters selected for the design of the inner rotor in order to meet these criteria have resulted in profiles having edges (discontinuities in slope) which, during use of the rotor in the pump, result in a bearing stress (Hertz stress) at the edge portion which increases to promote wear or settling thereat, thereby resulting in eventual deterioration in pump performance causing vibrations and/or noises. If, as sometimes occurs when the rotor designer becomes aware of the edge portion during the design stage, the edge portion is "smoothed over" during the design stage, and the resulting rotor will not operate at its maximum efficiency, but rather, will operate from the start as if the edge portion had become worn during use.
The present invention has been designed to overcome this problem, to provide a rotary device of the type described above in which the outer rotor is of the type described above with inwardly extending circularly arched teeth and the inner rotor is formed with an outer profile which is an inner envelope of the epitrochoidal curve, but without any edge portions.